Conventionally, in a method of manufacturing a steering rack, rack teeth have been formed by cutting a blank. However, in recent years, steering racks tend to be formed by plastic working using a die for the purpose of reducing cost.
For example, JP-A-10-58081 and JP-A-2001-79639 disclose methods of manufacturing a rack, in which a solid round bar is used as a rack blank, excess thickness parts which have been excluded while forming rack teeth are protruded toward a side of the rack as burrs, and the protruded burrs are removed by pressing or machining. However, this manufacturing method requires an extra step for eliminating the burrs so that the processing cost becomes high. Further, because the excess thickness parts are excluded to the side part as the burrs, a load to be applied to a die is large. In addition, because a blank material runs to a side of the burrs, the material is liable to be insufficiently filled into the rack teeth.
JP-B2-3442298 discloses another method of manufacturing a rack, in which a rack blank in a form of a solid round bar is finely processed by turning to have a rack teeth forming portion, and excess thickness parts, which are excluded while forming rack teeth, are received inside a run-off absorbing portion formed on a rear side of a die opposite to the rack teeth in a multistep in order to prevent burrs from being formed. However, according to this manufacturing method, the shape of the die is complex. Further, the die is of a closing type so that a stress applied to the die is large. Thus, the lifetime of the die is short, and the manufacturing cost of the die is high. In addition, because the shape of the rear side opposite to the rack teeth has a multistep configuration and is complex, a shape of a rack guide (see a component designated by a reference numeral 21 in FIG. 12 of JP-A-2004-34829), which guides the rear side with respect to the rack teeth, becomes complex. Thus, there has been a disadvantage in that the cost of a steering apparatus becomes high.
Moreover, in the manufacturing methods disclosed in JP-A-10-58081, JP-A-2001-79639, and JP-B2-3442298, the rack blanks are in a form of the solid round bar. Thus, a portion other than the rack teeth forming portion is solid and heavy. Therefore, when drilling a hole to such a portion so as to lighten the rack, the processing cost for drilling a hole is added. Furthermore, because the rack blank is heated in order to sufficiently fill the material within the rack teeth, there has been a disadvantage in that accuracy in forming the rack teeth is degraded.
JP-B2-3-5892, JP-B2-2928427, U.S. Pat. No. 6,575,009 B2, and JP-A-2002-178095 disclose some other methods of manufacturing a rack, in which rack teeth are formed by pressing a mandrel into a bore hole of a hollow tubular rack blank. However, in such a manufacturing method, because a large processing force is required for stroking the mandrel, a plurality of mandrels having sizes that are slightly different from each other are exchangingly used. Thus, a processing time is long, and a dedicated processing machine is needed. Accordingly, the manufacturing cost of the rack is high. Further, because a thickness of the bottom part of the rack teeth is thin, there has been a disadvantage in that the strength of the rack teeth is deteriorated.